Assume that a group of market variables change as they did during one of the days in a historical reference period, and apply these changes to the current values of these variables, which are then used to calculate asset values.

Assume that the values of the assets in the portfolio experience the same percentage change as they did during one of the days in a historical reference period.

Assume that a group of market variables has a multivariate normal distribution based on their movements during a historical reference period, and use a sampled value from this distribution to calculate asset values.

Assume that the values of the assets in the portfolio have a multivariate normal distribution based on their movements during a historical reference period, and then sample once from this distribution of asset values.